Dry reforming of methane over nickel-tantalum supported on fibrous ZSM-5 catalyst for production of synthesis gas

Abstract

Dry reforming of methane (DRM) is an environmentally benign process for production of synthesis gas carbon monoxide (CO) and hydrogen (H2) with low H2:CO ratio by utilization of carbon dioxide (CO2) and methane (CH4) as feed gas. The large-scale production of syngas via DRM is still in its infancy due to operational constraints exhibited by the several catalysts involved. In this study, microemulsion engineered fibrous ZSM-5 (FZSM-5) support was selected as the support material due to its extended surface area and stabilization of metal particles. In addition, nickel (Ni) loaded on FZSM-5 was prepared by double solvent, physical mixing and wetness impregnation methods. Furthermore, magnesium (Mg), calcium (Ca), tantalum (Ta) and gallium (Ga) promoters were added to Ni/FZSM-5 catalyst using wetness impregnation method. The catalysts were characterized using X-ray diffraction, nitrogen adsorption-desorption isotherm, transmission electron microscope, field-emission scanning electron microscope, Fourier-transform infrared spectrometer, IR-lutidine chemisorption, temperature-programmed desorption with ammonia and CO2, temperature-programmed reduction with H2, energy-dispersive X-ray, X-ray photoelectron spectrometer, Raman spectrometer, and thermogravimetric analysis. The effects of active metals, Ni-loading methods, support morphology, promoters, Ni-Ta ratio towards the activity, selectivity and stability of the Ni based catalysts were examined in DRM over a temperature range of 500–800 oC and atmospheric pressure. Results revealed that Ni species are highly active for dissociation of the reactants. Ni/FZSM-5 produced superior performance than conventional ZSM-5 supported Ni catalyst. High basicity, surface area and mesoporosity were responsible for the outstanding performance of FZSM-5 supported catalyst. The wetness impregnation catalyst produced superior performance, which was correlated to microscopic dispersion and low surface acidity. The activity of the bimetallic catalysts was in the order: Ni-Ga/FZSM-5 (CH4= 50.1 %, CO2= 58.8 %) < Ni-Ca/FZSM-5 (CH4= 82.9 %, CO2= 82.7 %) < Ni-Mg/FZSM-5 (CH4= 86.7 % ,CO2= 92.3 %) < Ni-Ta/FZSM-5 (CH4= 91 % ,CO2= 97.4 %). The side reaction (methane cracking, Boudouard and RWGS) test results indicated that Ni catalyst had high inclination towards methane cracking reaction. The presence of small Ta cations in Ni catalyst was enough to suppress the driving force for agglomeration and coke formation. The optimum CH4 conversion predicted from the response surface analysis was 96.6 % at reaction temperature of 784.15 °C, CO2:CH4 feed ratio of 2.52, and GHSV of 33,760 mL g-1 h-1. Experiment carried out with these optimum parameters gave 95.8 % CH4 conversion with error of 0.8 %. The strong catalytic stability of Ni-Ta/FZSM-5 was due to the small-size and immobilized Ni sites, enhanced reducibility and interaction of catalyst components. This study highlighted the contribution of fibrous structured ZSM-5 support and Ni-Ta catalyst in the quest for potent catalyst development for industrial production of syngas via DRM.